In accordance with the desire to be able to drive magnetic tape or film at high speeds both in the forward and reverse directions, the prior art has designed machines which are capable of feeding record film through a read/write station at very high speeds with the ability to reverse direction of the film feed in very short periods of time, all without imposing any excessive stresses on the film. In general, these machines have the film wound around rotating capstans used to draw the film from vacuum columns and feed it past the read/write station. A position sensor is placed in each vacuum column which is responsive to the position of the respective film loop for maintaining a relatively stable position of its film loop within the column. These film loop sensing means are generally spaced at predetermined maximum and minimum positions. Thus, as the capstan withdraws film from one vacuum column and the total length of the film stored therein is reduced to the predetermined minimum, control means associated with the film loop sensitive means initiates rotation of the related reel at some acceleration rate to feed film into the vacuum column. As soon as the film exceeds the predetermined maximum length, the loop sensitive means located at this predetermined maximum position instructs the associated control means to brake the reel to a stop. During this time, the film leaving the drive capstan is directed past the read/record station and into the other vacuum column. When the film therein reaches a predetermined maximum length, its associated film loop sensing means instructs the associated control means to initiate rotation of its related reel to remove the excess film from the vacuum column by winding it onto the take-up reel. Likewise, if the film loop length is reduced to the predetermined minimum, associated control means brakes the take-up reel to terminate the winding operation.
When sudden reversals of the direction of film travel are required, such when the machine is required to switch from a forward drive to a rewind operation, the vacuum loops yield or take-up film practically instantaneously since the inertia of the film in the path from vacuum loop to vacuum loop over the read/record station is extremely low. After the initial very short interval of time, tension is restored to the film and the supply and take-up reels both return to normal operation.
Although the machines described hereinabove perform well when the film or tape is driven at continuous high speeds, a problem still remains when it is desired to have, in addition to the continuous features described hereinabove, a non-continuous or intermittently driven system having a variable film advance control. For example, in the computer output microfilm (COM) systems utilizing electron beam recording (EBR), characters are written directly onto unexposed film. As set forth hereinabove, the unexposed film is drawn into the vacuum chamber and a page, or frame, is written one character at a time onto the film. After the information is written by the electron beam, the film is incremented and the next page written. When viewed by a microfilm reader, the information on each frame appears exactly as it would on a regular printed page.
Typically, the standard output format from a computer is 132 characters per line and 64 lines per page. However, applications exist where more than 132 characters per line and 64 lines per page would be desirable. Although some COM systems provide this capability, the techniques generally involve complex mechanical and/or electronic components and the added cost of this feature as a COM system option generally makes the option unattractive.